A type of in-vehicle image display apparatuses is known as head-up displays (HUDs). A HUD uses an optical system to reflect a light beam generated by a light source, such as a liquid crystal display (LCD) and a CRT display, to form an image on a reflection member such as a windshield and projects the reflected light beam to an observer (driver/operator). The observer can simultaneously view the background transmitted through the reflection member and the image presented by the light beam from the light beam generator (source), the image being superimposed on the background.
It is desirable for a HUD to match the surrounding environment observed as a background with the image by the light beam from the light beam generator (source).
To apply the image display apparatus to in-vehicle car navigation, there have been some attempts to present an image showing routing information of the vehicle or the like while the image is being matched with a background including a road. For example, one such attempts is a method which changes the form of an arrow showing the routing information at a corner of a road (refer to JP-A2006-284458). Another is a method which changes the size of an arrow located at a spatial position of the surrounding environment according to the vehicle speed (refer to JP-A 2006-17626).
In a conventional HUD, the light beam from the light beam generator is reflected by the reflection member and projected onto the eyes of an observer, to be viewed with both eyes. When the image is viewed by both eyes, the position at which the observer perceives the image to be located (perceived position) is the same as the position at which the image is presented as a virtual image (virtual image position) due to binocular disparity. It is therefore difficult to match the perceived position with distant spatial positions.
On the other hand, a monocle HUD has been proposed in which the light beam from the light beam generator is incident on and perceived by a single eye of the observer. In the monocle HUD, the perceived position of the image is not fixed to the virtual image position because there is no binocular disparity. Accordingly, the image can be located at any perceived position, and the perceived position of the image can be matched with the spatial positions of the surrounding environment.
However, in the presence of an obstacle such as a wall or a vehicle in the background, the perceived position of the image is fixed to the position of the obstacle, and it is difficult to locate the perceived position of the image farther than the obstacle. As described above, the perceived position of the image is restricted depending on the presence of the obstacle, making it difficult to view the image.
Patent Citation 1
    JP-A 2006-284458 (KOKAI)Patent Citation 2    JP-A, 2006-17626 (KOKAI)